Take a shot at broadleaf weeds

Steve Hart

As we move into the spring of 2001, you will find an increasing number of herbicides available for post-emergence broadleaf weed control. At times, the number of options may seem overwhelming. However, familiarizing yourself with the characteristics of a few active ingredients will help you choose the herbicide combinations that offer the greatest benefit to your weed-control programs.

Many applicators take broadleaf herbicides for granted. Considering that the oldest have been around since the 1940s, perhaps this is understandable. However, the ability to selectively remove broadleaf weeds without eliminating the desirable turf is a nearly indispensable aspect of turf care. Imagine what most turf would look like without this possibility. We are fortunate indeed to have such chemical tools available to us.

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Most post-emergence broadleaf herbicide combination products contain two or more of the following active ingredients: 2,4-D, dichlorprop, MCPP, MCPA, dicamba, triclopyr and clopyralid. All of these herbicides are considered "growth regulating" and have similar modes of action. When absorbed by the foliage of broadleaf weeds, these herbicides act as synthetic auxins (hormones) and overstimulate growth. After treatment, you may notice abnormal growth (referred to as epinasty), which eventually leads to the death of the plant.

Although these herbicides all work in a similar way, effectiveness on specific broadleaf weeds varies. This is due to the ability of some broadleaf weeds to metabolize one or more of these herbicides before the abnormal growth becomes lethal to the plant. To counter this, manufacturers offer a variety of combination products. These have become the most widely used options for broadleaf weed control.

Relatively new chemistries such as quinclorac, triclopyr and clopyralid have been added to the arsenal, and have created an even greater range of control options. Thus, combinations will continue to dominate the battle against broadleaf weeds.

Finding the form that fits
The activity and effectiveness of herbicides not only depends on the active ingredient, but also formulation. Herbicides may be formulated as liquids, powders, concentrates, granules or some other form. However, most are formulated for application as liquids.

Some broadleaf herbicides, especially fertilizer/herbicide combination products, are granular. Generally, liquid applications are more effective so they are the primary formulation used by professional turf managers. Granules do have advantages, however. They can be easier to handle and apply, and like all granular products, they allow you to cover a large area in a short time. Also, the effectiveness of granular formulations has been limited by getting the herbicide granules to stick to the foliage of the weeds. However, granular technology is making strides and this concern is no longer as restrictive as it once was.

Formulation also affects weed control by whether the active ingredient is formulated as an ester or a salt. The most popular salt formulation is the amine salt, but some products contain other salts. Because salt formulations are relatively nonvolatile, they are preferred for application during warm weather.

Ester formulations provide better penetration of leaf surfaces. Therefore, they tend to be more effective. However, ester formulations are more volatile than salts. Because volatility may put nearby ornamentals at risk, many turf applicators restrict their use of esters to cool weather, such as during spring and fall. However, several manufacturers have developed ester formulations with lower volatility (often denoted by "LV" or "L.V." in the product name), expanding the traditionally narrow application window of esters. Some common ester formulations you will encounter are isocotyl and butoxyethyl esters.

The following discussion lists the primary active ingredients in the most widely used broadleaf herbicides, including some newer and older, non-phenoxy chemicals. This information should help you develop "the big picture" of weed-control options and assist in making product choices.

Primary active ingredients
- 2,4-D. Introduced in the early 1940's, 2,4-D is the oldest, most widely used turfgrass herbicide. 2,4-D is a phenoxy acid herbicide that is particularly effective for control of perennial weeds with taproots such as dandelion, broadleaf plantain, mustards and shepherdspurse. Amine formulations are most common, but the low-volatile ester formulations of 2,4-D are recommended for control of wild garlic and wild onion. Low-volatile ester formulations generally work better than amine formulations in early spring and fall, but you should avoid using them when air temperature approaches 85F.

White clover, chickweed, purslane and ground ivy are not as susceptible to 2,4-D. You should limit or avoid the use of 2,4-D on newly seeded turf, bentgrass, centipede and St. Augustine grass. However, some manufacturers do formulate 2,4-D products that are safe for use on some of these sites. (Check the product label for specific instructions.)

- MCPA and MCPP. These also are phenoxy acid herbicides. They are similar to each other in chemical structure and the spectrum of broadleaf weeds they control. Manufacturers often combine them with 2,4-D, or use them as a 2,4-D substitute in prepackaged mixtures. Neither MCPA nor MCPP is as active on as many weeds as 2,4-D. However, both herbicides are active on important broadleaf weeds such as clovers and chickweeds. Also, there are many formulations labeled for use on sensitive areas like bentgrass greens. Amine formulations are most commonly used, though ester formulations are available.

- Dicamba. Dicamba is a benzoic acid herbicide with a mode of action similar to the phenoxy herbicides. It controls many weeds, several of which are not as susceptible to 2,4-D or MCPP. Of particular importance are certain summer annual weeds that have a prostrate growth habit, including knotweed, purslane and spurge.

Dicamba also is more effective than the phenoxy-acid herbicides on ground ivy. However, many herbicide combinations do not contain high enough concentrations of dicamba for control of problem weeds such as ground ivy. In these cases, you can use dicamba alone at labeled rates. Two common turf weeds, buckhorn and broadleaf plantains, are not controlled well by dicamba.

Carefully follow label instructions when using dicamba on bentgrass, centipedegrass and St. Augustinegrass. They are fairly sensitive to this chemical.

- Dichlorprop (2,4-DP or DCPP). Dichlorprop is only sold in combination with 2,4-D and other herbicides. Combinations of this herbicide with 2,4-D will more effectively control many weeds, such as henbit, knotweed and spurge, than 2,4-D alone. Both amine and ester formulations are available.

- Triclopyr. Triclopyr is a relatively new herbicide based on a chemical structure known as a pyridine. Classified as a pyridinoxy acid, triclopyr does not have a spectrum of control equal to 2,4-D, MCPA or MCPP. However, it does control many weeds that are not very susceptible to these herbicides. Of all the broadleaf herbicides available, triclopyr has the highest levels of activity on ground ivy and oxalis.

Triclopyr is available alone but is often sold in combination with 2,4-D, MCPA or clopyralid. You should avoid the use of triclopyr alone at high application rates on all warm-season grasses and bentgrass. However, when used at low rates in combination with other herbicides, triclopyr may be used in many warm-season grasses.

- Clopyralid. Another relatively new pyridinoxy-acid herbicide, clopyralid is available alone but is generally sold in combination with other herbicides. As with triclopyr, you will notice an increase in the number of herbicide combinations available containing clopyralid. Although the weed control spectrum of clopyralid is not as great as other broadleaf herbicides, it will provide effective control of some key weeds such as clover, thistle and pineapple weed. When used alone, clopyralid is safe for use on most cool- and warm-season grasses.

- Quinclorac. This unique chemical, registered for turf use in 1998, is in the quinolinecarboxylic acid family of herbicides. Quinclorac's primary use is for post-emergence crabgrass control. However, it effectively controls a few important broadleaf weed species including white clover, veronica and dandelions. This versatility allows you to simultaneously extend the window of crabgrass control and control certain problematic broadleaf weeds. No combination products are available with quinclorac, but you can tank-mix it with important broadleaf herbicides like 2,4-D, triclopyr and clopyralid, MCPA, MCPP and others to broaden the spectrum of control.

Introductions for 2001
Several new herbicides are available to the turf and ornamental market for controlling broadleaf weeds this year. Re-releases of the sulfonylurea chemistry (metsulfuron methyl and chlorsulfuron) and new combinations of older chemistries are most notable.

- Metsulfuron methyl is in the sulfonylurea class of herbicides that inhibit growing points (meristematic regions) and provide good control at low rates. Using metsulfuron methyl, you can control a wide range of broadleaf weeds including white and yellow clover, ground ivy, prostrate spurge and oxalis.

Following applications, seeding should be delayed for 2 months. Metsulfuron is for use on Kentucky bluegrass, fine fescue, bermudagrass, St. Augustinegrass, zoysiagrass (some varieties) and centipedegrass. This herbicide should not be used on turf that has been stressed or on cool-season grasses when temperatures are above 85F.

- Chlorsulfuron is another sulfonylurea herbicide. Labeled as a spot treatment on most turf sites, you may broadcast spray it on golf courses. You can control a large number of broadleaf weeds and suppress wild violets with chlorsulfuron. It will also control perennial bunch or clump grasses like tall fescue and perennial ryegrass. Only apply to Kentucky bluegrass, fine fescue, bentgrass (when mowed over 1/2 inch in height), bahiagrass and bermudagrass lawns. Chlorsulfuron can be tank-mixed with many other broadleaf herbicides to broaden the spectrum of control.

- DSMA + 2,4-D combines the broadleaf weed herbicidal properties of 2,4-D with the grassy weed herbicidal properties of the arsenical class of herbicides in which DSMA is grouped. You can use this combination to control some grassy weeds while achieving good control of broadleaf weeds.

- Ammoniated soap of fatty acids is a herbicidal soap with rapid contact activity. This herbicide reacts with aboveground plant parts, but does not kill roots. Therefore, it is most effective on annual weeds. However, regular applications can suppress perennials. It is a non-selective herbicide with little residual so seeding can be initiated 5 days after treatment.

- MCPA + clopyralid + dichlorprop combines phenoxy chemistry (MCPA and dichlorprop) with pyridinoxy chemistry (clopyralid). This combination offers broad-spectrum control with the addition of clopyralid for some of the harder to control weeds.

- MCPA + clopyralid + triclopyr is another combination of phenoxy and pyridinoxy chemistries. This combination offers you control of catsear, groundsel, kudzu, nightshades and a few other hard-to-control weeds that some combination products do not control.

Choosing the right chemical or combination for your broadleaf weed management program starts with proper weed identification. Keep in mind that the weed species I mentioned here are just some of the notables. All the chemicals I discussed in this article control many other weed species. The weeds present at your particular site and the safety of chemicals on your specific type of turfgrass will be the main criteria for deciding which chemical or chemical combination is best for you. Use the Herbicide Update on page 49 as a guide to assist you in the selection of the proper herbicide or herbicide combination.

There is little doubt that we will continue to rely on older phenoxy and benzoic acid chemistry to control the majority of broadleaf weeds. However, newer technologies such as the pyridinoxy acid (triclopyr and clopyralid) and the quinolinecarboxylic acid (quinclorac) herbicides will continually increase our arsenal of control products. More new products are on the horizon. They will probably target specific hard-to-control weeds and lend themselves to tank-mixing to achieve the broad-spectrum control that has become today's standard.